For the last several years, anti-reflective composites have been used on an ever expanding basis for a myriad of purposes. Anti-reflective composites are most commonly used on windows, mirrors, and an assortment of display applications which includes television screens and computer monitor screens to minimize reflective "glare." The most common design for such composites is one having a quarter-wave optical thickness at a particular wavelength. This design is capable of reducing reflectance of a surface to less than 1% over the visible range.
A typical anti-reflective composite consists of a light transmissive substrate and one or more transparent anti-reflective top layers. A transparent hard coat layer is often deposited between the substrate and the anti-reflective layers to give the composite both mechanical durability and physical strength. The materials used in each layer and the thicknesses of each layer are chosen so that a maximum amount of light is transmitted through the composite while a minimum amount of light is reflected by the composite.
Numerous anti-reflective composite designs are known to date, most of which are comprised of high and low refractive index materials in pairs. One of the earliest patents in this field, i.e., U.S. Pat. No. 2,478,385, describes a three-layer structure of medium/high/low refractive index materials over a glass substrate.
Another earlier patent dealing with anti-reflective coatings is U.S. Pat. No. 3,432,225, wherein is disclosed a method of combining a four-layer anti-reflective coating system using two different materials, i.e. ZrO.sub.2 and MgF.sub.2. A basic problem with this approach is the inherent softness of MgF.sub.2, which limits the use of this approach in many applications.
Another multi-layer anti-reflective coating is disclosed in U.S. Pat. No. 3,565,509 wherein a three layer system is reduced to two using two materials for production simplicity.
The most common anti-reflective coating is a four layer structure. Such a design can be made from two anti-reflective coating materials rather than from three or four different materials as indicated in some earlier patents. The thickness of each layer is optimized to obtain the best properties over a broad range of the spectrum.
Most anti-reflective composite designs utilize high index dielectric anti-reflective layers as part of the construction. In applications where electromagnetic shielding and static discharge control are of primary concern, conductive high index oxides such as indium oxide or tin oxide are incorporated in the design structure. U.S. Pat. No. 4,422,721 covers the use of conductive coatings as part of the anti-reflective design structure.
U.S. Pat. No. 5,170,291 discloses a four-layer anti-reflective composite where DC reactive sputtering has been suggested as a preferred method of deposition. U.S. Pat. No. 5,579,162 discloses a multi-layer anti-reflective composite utilizing DC reactive sputtering as a preferred method of deposition for temperature sensitive substrates.
There are several problems with the anti-reflective composites presently known. A first problem is that special processes are required to deposit anti-reflective coatings onto a polymeric substrate.
A second problem is that most production techniques proposed for deposition of various layers of anti-reflective composites are possible, but few are practical. This is particularly important since there is no documented method of depositing anti-reflective coatings in a continuous roll coating (web) system.
A third problem with presently known anti-reflective composites is the general lack of manufacturing efficiency and low deposition rates previously inherent in the manufacture of anti-reflective composites.
A fourth problem with presently known anti-reflective composites is the great difficulty in being able to adhere anti-reflective coatings onto a polymeric substrate, especially a polymeric substrate which is covered with a hard coat.
Accordingly, there is a need for an improved anti-reflective composite which overcomes these problems in the prior art.